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For intermittent use, thermal mass is the real killer. Just getting that slab up to temperature is going to take about 10,000 btu per degree F., and a deeper soil mass will take much more. It's great to have that mass if you keep a constant temperature (eg passive solar), but if you're going through cycles of heating it and then letting it cool fully the lighter mass will cost less.
Some kind of lightweight, insulative floor covering might be a good bet.
If you look closely, you'll see that it's a knot remover too!
I wonder if this would work for misdrilled holes. I had to return my reversible drill because the reversing feature didn't work -- the hole was still there.
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